1. Field of the Invention
This invention relates to a drive circuit for a vehicle occupant safety apparatus such as an air-bag apparatus or a vehicle safety-belt pretensioner (tightner). Also, this invention relates to a constant-current control circuit usable in various circuits such as a drive circuit for a vehicle occupant safety apparatus.
2. Description of the Related Art
A known vehicle air-bag system includes air bags and squibs for activating the air bags respectively. The squibs are connected to an electric power supply via resistors respectively. An energy storage backup capacitor is connected in parallel to the electric power supply. The squibs are arranged in parallel to each other. When one of the squibs falls into a short-circuited state, the arrangement of the resistors connected to the squibs enables the other squibs to receive necessary drive currents from the electric power supply and the backup capacitor. However, the resistors consume certain amounts of electric power, requiring greater power feeding capabilities of the electric power supply or the backup capacitor.
U.S. Pat. No. 5,135,254 corresponding to Japanese published unexamined patent application 4-2544 discloses a vehicle air-bag apparatus in which squibs are connected via respective constant-current circuits to a vehicle battery and a backup capacitor. The constant-current circuits serve to regulate drive currents (firing currents) to the squibs at constant levels, respectively. Each of the constant-current circuits has a current sensing resistor for generating a voltage thereacross which is proportional to a current flowing through a related squib. The current sensing resistor is disposed in a current flow path to the related squib. Therefore, the current sensing resistor consumes a portion of electric power fed toward the related squib.
U.S. Pat. No. 5,204,547 corresponding to Japanese published unexamined patent application 4-500641 discloses an air-bag system for protecting the occupants of a motor vehicle in the event of a collision. The air-bag system has a plurality of firing circuits comprising air-bag igniters in series with respective power transistors which can be triggered in the event of an accident to inflate a corresponding plurality of air bags. A single energy storage capacitor is connected to all of the firing circuits for supplying energy to activate the air-bag igniters in the event that there is a loss of a battery voltage when the system has been actuated. A comparator monitors the voltage drops across respective resistors (current sensing resistors) in series with the power transistors, and reduces the currents through these transistors in the event that excessive currents are detected. Steps are taken to ensure that the period of activation of the current supply to the air-bag igniters is limited. Each of the current sensing resistors is disposed in a current flow path to the related air-bag igniter. Therefore, the current sensing resistor consumes a portion of electric power fed toward the related air-bag igniter.